1. Field of the Invention
The present invention is directed to a method for producing merchant grade hydrogen and carbon monoxide from a steam reformed hydrocarbon feed mixture. More particularly, the present invention is directed to a method for producing hydrogen and carbon monoxide from a feed mixture comprising hydrogen, carbon monoxide, carbon dioxide, and methane.
2. Description of the Prior Art
Various methods are known for separating gaseous mixtures produced by the steam reforming of hydrocarbons. Steam reforming to produce hydrogen consists of treating a hydrocarbon feed mixture with steam in a catalytic steam reactor (reformer) which consists of a number of tubes placed in a furnace at a temperature in the range from about 1250.degree. F. to about 1700.degree. F. The reversible reforming reactions which occur when methane is used as the hydrocarbon feed mixture are set out below. EQU CH.sub.4 H.sub.2 O=Co+3H.sub.2 EQU CH.sub.4 +2H.sub.2 O=CO.sub.2 4H.sub.2 EQU CO+H.sub.2 O=CO.sub.2 +H.sub.2
Carbon monoxide and carbon dioxide are generally removed by shift conversion (reaction of carbon monoxide with steam to form additional hydrogen and carbon dioxide), absorption in amines or other alkaline solvents (carbon dioxide removal), and methanation (conversion of trace carbon monoxide and carbon dioxide to methane). When carbon monoxide is a desired product, the shift conversion and methanation steps are not employed.
The hydrogen-rich gas mixture exiting the steam reformer consists of an equilibrium mixture of hydrogen, carbon monoxide, carbon dioxide, water vapor, and unreacted methane. The reforming reactions are endothermic and therefore hydrocarbons and process waste gases are burned in the reformer furnace to provide the endothermic heat.
Hydrocarbon steam reforming reactions and hydrogen separation processes are disclosed in more detail in "Ammonia and Synthesis Gas: Recent and Energy Saving Processes", Edited by F.J. Brykowski, Chemical Technology Review No. 193, Energy Technology Review No. 68, Published by Noyes Data Corporation, Park Ridge, New Jersey, 1981, which disclosure is incorporated herein by reference.
Conventional methods for recovering hydrogen and carbon monoxide from a hydrocarbon steam reformed feed mixture have generally focused on cryogenic distillation processes to separate and purify hydrogen and carbon monoxide in the mixture after carbon dioxide is removed. Cryogenic separation processes tend to have a high capital cost especially when more than one pure product is required.
U.S. Pat. No. 4,778,670, issued to Pinto, discloses a pressure swing adsorption process for producing technical hydrogen which comprises passing a raw gas containing a specific ratio of hydrogen, nitrogen, and carbon oxides to a pressure swing adsorbent and stopping the flow of feed gas in the cycle when the integrated nitrogen content of the unadsorbed product gas of the pressure swing adsorption stage is in the range of 1% to 10% by volume.
German patent application no. 3,427,804, to Linde A.G., discloses a process for reforming a hydrocarbon with carbon dioxide to obtain a gas mixture comprising hydrogen, carbon monoxide, and carbon dioxide and separating the mixture into separate streams of hydrogen, carbon monoxide, and carbon dioxide. The methods for purifying the hydrogen and carbon monoxide streams are not disclosed.
Methods for separating hydrogen and carbon monoxide by pressure swing adsorption processes are disclosed in European patent application no. 317,235A2, to Krishnamurthy et al, and the references cited therein. Krishnamurthy et al. discloses a method for forming hydrogen and carbon monoxide from a feed mixture exiting a hydrocarbon steam reformer comprising hydrogen, carbon monoxide, and carbon dioxide. The method comprises the steps of passing the feed mixture through a sorptive separation to separate a hydrogen product, a carbon monoxide-rich product, and a carbon dioxide-rich product. The carbon monoxide-rich product is further purified in a two stage pressure swing adsorption system. The first stage comprises an activated carbon adsorbent which removes carbon monoxide and methane as the strongly adsorbed waste stream. The second stage comprises a zeolite adsorbent and produces a pure carbon monoxide stream as an adsorbed product.
U.S. Pat. No. 4,917,711, issued to Xie et al., discloses an adsorbent for carbon monoxide and unsaturated hydrocarbons which comprises a high surface area support, such as a zeolite, alumina, silica gel, aluminosilicate, or aluminophosphate, and cuprous or cupric compound. The adsorbent may be used to separate carbon monoxide and unsaturated hydrocarbons from a gaseous mixture containing hydrogen, nitrogen, argon, helium, methane, ethane, propane, and carbon dioxide by passing the mixture through the adsorbent and releasing the adsorbed carbon monoxide by heating, or lowering the pressure of, the adsorbent.
Japanese patent JP01203019 discloses a four column pressure swing adsorption system for separating carbon monoxide from a gaseous mixture. The columns contain an adsorbent containing copper to adsorb carbon monoxide gas.
U.S. Pat. No. 4,914,076, issued to Tsuji et al., discloses a method for preparing an adsorbent for carbon monoxide which comprises contacting an alumina or silica-alumina support with a mixed solution or dispersion of a copper (II) salt and a reducing agent, and then removing the solvent.
U.S. Pat. No. 4,783,433, issued to Tajima et al., discloses an adsorbent for separating carbon monoxide from a gaseous mixture containing carbon dioxide which comprises a zeolite resin with a silica/alumina ratio of not more than 10, in which not less than 50% of the cation exchange sites have been replaced by Cu(I) ions, in the pores of which, one or more salts of the metals Cu(I), Fe, Zn, Ni, and or Mg are dispersed.
Japanese patent JP61242908 discloses an adsorbent for carbon monoxide Which is prepared by supporting a copper (I) compound on an activated carbon support wherein the volume of pores having a diameter of under 10 angstroms is under 0.33 ml/g.
U.S. Pat. No. 4,743,276, issued to Nishida et al., discloses an adsorbent for carbon monoxide which comprises a zeolite resin with a silica/alumina ratio of not more than 10, in which not less than 50% of the cation exchange sites have been replaced by Cu(I) ions, in the pores of which, one or more salts of the metals Cu(I), Fe, Zn, Ni, and or Mg are dispersed.
In a pressure swing adsorption system (PSA), a gaseous mixture is passed at an elevated pressure through a bed of an adsorbent material which selectively adsorbs one or more of the components of the gaseous mixture. Product gas, enriched in the unadsorbed gaseous component(s), is then withdrawn from the bed.
The term "gaseous mixture", as used herein, refers to a gaseous mixture, such as air, primarily comprised of two or more components having different molecular size. The term "enriched gas" refers to a gas comprised of the component(s) of the gaseous mixture relatively unadsorbed after passage of the gaseous mixture through the adsorbent bed. The enriched gas generally must meet a predetermined purity level, for example, from about 90% to about 99%, in the unadsorbed component(s). The term "lean gas" refers to a gas exiting from the adsorption bed that fails to meet the predetermined purity level set for the enriched gas. When the strongly adsorbed component is the desired product, a cocurrent depressurization step and a cocurrent purge step of the strongly adsorbed component are added to the process.
The term "adsorption bed" refers either to a single bed or a serial arrangement of two beds. The inlet end of a single bed system is the inlet end of the single bed while the inlet end of the two bed system (arranged in series) is the inlet end of the first bed in the system. The outlet end of a single bed system is the outlet end of the single bed and the outlet end of the two bed system (arranged in series) is the outlet end of the second bed in the system. By using two adsorption beds in parallel in a system and by cycling (alternating) between the adsorption beds, product gas can be obtained continuously.
As a gaseous mixture travels through a bed of adsorbent, the adsorbable gaseous components of the mixture enter and fill the pores of the adsorbent. After a period of time, the composition of the gas exiting the bed of adsorbent is essentially the same as the composition entering the bed. This period of time is known as the breakthrough point. At some time prior to this breakthrough point, the adsorbent bed must be regenerated. Regeneration involves stopping the flow of gaseous mixture through the bed and purging the bed of the adsorbed components generally by venting the bed to atmospheric or subatmospheric pressure.
A pressure swing adsorption system generally employs two adsorbent beds operated on cycles which are sequenced to be out of phase with one another by 180.degree. so that when one bed is in the adsorption or production step, the other bed is in the regeneration step. The two adsorption beds may be connected in series or in parallel. In a serial arrangement, the gas exiting the outlet end of the first bed enters the inlet end of the second bed. In a parallel arrangement, the gaseous mixture enters the inlet end of all beds comprising the system. Generally, a serial arrangement of beds is preferred for obtaining a high purity gas product and a parallel arrangement of beds is preferred for purifying a large quantity of a gaseous mixture in a short time cycle.
Between the adsorption step and the regeneration step, the pressure in the two adsorption beds is generally equalized by connecting the inlet ends of the two beds together and the outlet ends of the two beds together. During the pressure equalization step, the gas within the pores of the adsorption bed which has just completed its adsorption step (under high pressure) flows into the adsorption bed which has just completed its regeneration step (under low pressure) because of the pressure differential which exists between the two beds. The adsorption bed which completed its adsorption step is depressurized and the adsorption bed which completed its regeneration step is repressurized. This pressure equalization step improves the yield of the product gas because the gas within the pores of the bed which has just completed its adsorption step has already been enriched. When more than two beds are employed in the adsorption system, it is common to have a number of pressure equalizations steps.
Gas separation by the pressure swing adsorption method is more fully described in, for example, "Gas Separation by Adsorption Processes", Ralph T. Yang, Ed., Chapter 7, "Pressure Swing Adsorption: Principles and Processes" Butterworth 1987, and in U.S. Pat. Nos. 2,944,627, 3,801,513, and 3,960,522, which disclosures are incorporated by reference herein. Modifications and improvements in the pressure swing adsorption process and apparatus are described in detail in, for example, U.S. Pat. Nos. 4,415,340 and 4,340,398, which disclosures are incorporated by reference herein.
While the above methods disclose processes for separating carbon monoxide, none of the methods disclose satisfactory processes for recovering both hydrogen and carbon monoxide from a hydrocarbon steam reformed feed mixture economically and in high purity. Methods for separating hydrogen and carbon monoxide from a hydrocarbon steam reformed feed mixture require multi-stage systems to purify carbon monoxide. Methods for separating carbon monoxide using copper exchanged sieves have focused on the separation of waste gases from steel mills which contain nitrogen, carbon monoxide, and carbon dioxide but not hydrogen. Conventional cryogenic separation processes tend to have a high capital cost especially when more than one pure product is required. The present invention provides an improved method for producing hydrogen and carbon monoxide from a hydrocarbon steam reformed feed mixture employing a novel combination of pressure swing adsorption methods which minimizes capital cost requirements and increases the recovery of carbon monoxide.